1. Field of the Invention
The present invention relates to a light scattering element, an optical scanning device, and an image displaying device which reduces speckle noise of laser light.
2. Description of the Related Art
Conventionally, there has been proposed, as one embodiment of an image display device, a projection display which displays an image on a screen, by projecting light from a light source to the screen while deflecting the same with a light deflector. As the light deflector, for example, a light deflector which integrally forms mechanical components such as a mirror and a piezoelectric actuator on a semiconductor substrate, has been proposed. This light deflector is one type of MEMS (micro electro mechanical systems) device using semiconductor process or micromachine technology (for example, refer to Japanese Patent Laid-Open No. 2005-148459). In this light deflector, one end of the piezoelectric actuator is coupled to a frame unit (supporting unit) and is supported thereby. The torque generated by the piezoelectric actuator is transmitted to a torsion bar (elastic beam) connected to the other end of the piezoelectric actuator, and rotary drives the mirror equipped at the leading end of the torsion bar. Such light deflector has a merit that large driving force may be obtained with a compact and simple structure.
In the projection display mentioned above, a technology which uses laser light source such as a semiconductor laser for the light source thereof has been proposed. The laser light source has long lifetime of the light source compared to a lamp, has high energy conversion efficiency, and has high chromatic purity of light's three prime colors. However, in the case where the laser light source is used as the light source, there arises a problem that speckle noise (granular interference pattern) is generated and the image quality becomes degraded. The speckle noise is a phenomenon generated when coherent light in which phase from the laser light source is uniform is scattered by a random (having fine irregularity) phase front (object surface), and the scattered light from adjacent region on the object surface interfere on a viewing screen. The speckle noise appears as a granular intensity distribution (spotty pattern) on the viewing screen.
In the projection display using laser light source, if such speckle noise is generated between the screen as the object surface and eyes (retinas) of an observer as the viewing screen, the observer recognizes the degradation of the image. That is, the speckle noise is superimposed on the image which significantly degrades the image quality. Various techniques has been proposed in order to reduce such speckle noise (for example, refer to Japanese Patent Laid-Open No. 2002-62582, Japanese Patent Laid-Open No. 2004-138669).
The image display device of Japanese Patent Laid-Open No. 2002-62582 reduces the speckle noise by spatially changing the deflecting state of incident laser light by a polarized light distribution changing means. To be more specific, the image display device displays the image by a light emitted from a light source and modulated by a space modulator, and converts a spatial polarized light distribution of the light emitted from the light source so that the polarization directions of the light input to the adjacent pixels of the space modulator becomes orthogonal to each other. Because polarized light orthogonal to each other does not interfere with one another, the speckle noise by the interference of regions corresponding to the adjacent pixels of the space modulator is suppressed in the displayed image.
On the other hand, the image display device of Japanese Patent Laid-Open No. 2004-138669 does not change the polarized state of incident laser light, but changes the scattering direction variously by inserting a light scattering element in a light path between the laser light source and the space modulator for image formation, and vibrating the light scattering element at high speed with a driving mechanism (vibrating means for applying vibration). By doing so, an approach for reducing a speckle contrast by averaging the speckle pattern has been made.
However, in the devices disclosed in Japanese Patent Laid-Open No. 2002-62582 and Japanese Patent Laid-Open No. 2004-138669, it is necessary to provide, in addition to the light deflector, the optical element or the driving mechanism to the optical system for reducing the speckle noise. More specifically, in the technology of Japanese Patent Laid-Open No. 2002-62582, it becomes necessary to provide the polarized light distribution changing means, and to expand a radius of laser light more than the size of a polarized light element of the polarized light distribution changing means. Therefore, either additional lens becomes necessary or the size of the optical system becomes larger. Further, in the technique disclosed in Japanese Patent Laid-Open No. 2004-138669, it is necessary to further add the driving mechanism such as the vibrating means for vibrating the light scattering element. As such, in the techniques such as those disclosed in Japanese Patent Laid-Open No. 2002-62582 and Japanese Patent Laid-Open No. 2004-138669, various problems arose such as increase in the number of components, increase in the size as the system, increase in power consumption, and burden of optical alignment.